The present invention relates generally to microwave mixer circuits and, more particularly, to a compact microstripline microwave mixer circuit utilizing a waveguide filter.
One of the difficulties in designing microwave mixer circuits is realizing correct terminations at the various signal frequencies within the mixer circuits. The frequencies of these signals is determined by a mixer's physical configuration, the desired radio frequency (RF) input signals, the local oscillator (LO) signal or signals and intermediate frequency (IF) output signals.
It is known in the art that the performance of a mixer circuit critically depends on the correct termination at the RF input, the LO input, and the IF output for a large number of frequencies. The frequencies of concern include not only the frequencies of the noted signals but also the sums and differences of those frequencies and their harmonics to some order, infinite in theory but finitely truncated in practice.
In microstripline mixer circuits, such terminations have taken the form of tuned stubs, either open ended or shorted, placed at suitable distances along the microstripline. In general, simple devices of this type serve to properly terminate the fundamental frequencies and possibly their odd harmonics; however, usually the many sums and differences of harmonics are not properly terminated. Thus, proper terminations on microstripline mixer circuits usually require multiple matching sections and stubs to cover several frequencies and bands, thereby taking up substantial space on the microstripline substrate. Waveguide mixers also require tuned lengths and terminations, which also tend to be large and space consuming.
Accordingly, there is a need for a microstripline microwave mixer circuit which provides good performance in a compact space.